1. Field of the Invention
The present invention relates to an optical waveguide device used for optical communication and an optical waveguide module modified from the same optical waveguide device. Particularly, the present invention relates to an optical waveguide device and an optical waveguide module having the functions for multiplexing/demultiplexing or coupling/decoupling optical signals to reduce optical loss thereof.
2. Description of Related Art
In optical communication, further expansion of transmission capacity is expected. Accordingly, a wavelength division multiplexing optical transmitting method has widely been used because of its comparatively easier expandability in the transmission capacity. In recent years, further improvement in the degree of wavelength multiplexing is requested in the wavelength division multiplexing optical transmitting method. As a key device to realize such method, an optical multiplexing/demultiplexing element has been proposed. An arrayed waveguide grating type optical multiplexing/demultiplexing element is formed in a passive structure to enable multiplexing/demultiplexing of a plurality of wavelengths. This element is characterized in low loss, high extinction ratio, and narrow-band pass.
A basic structure of a general arrayed waveguide grating of the related art is illustrated in FIG. 1. This general type arrayed waveguide grating of the related art is constituted by forming, on a substrate 51, a input waveguide array 52, an output waveguide array 53, a channel waveguide array (diffraction gating) 54, a first slab waveguide 55 for connecting the input waveguide array 52 and the channel waveguide array 54, and a second slab waveguide 56 for connecting the channel waveguide array 54 and the output waveguide array 53. The channel waveguide array 54 is set in its length to become sequentially longer with the predetermined difference ΔL in lengths of the waveguides. In this array waveguide grating, the wavelength multiplexing optical signals λ1 to λm (m is a positive integer) inputted from an end of the input waveguide array 52 are divided into optical signals λ1, λ2, . . . , λm of every wavelength and are then outputted from the end of the output waveguide array 53 in the opposite side. On the contrary, the optical signals λ1, λ2, . . . , λm of every wavelength inputted from the end of the output waveguide array 53 is multiplexed into the wavelength multiplexing signals λ1 to λm and are then outputted from the end of the input waveguide array 52 in the opposite side.
As explained above, the optical communication based on the wavelength multiplexing optical transmitting system is requested in these years to satisfy the requirements such as higher wavelength multiplexing rate, higher transmission rate, and longer transmission distance. Accordingly, the array waveguide grating has been expected to realize increase in the transmission distance and further reduction in transmission loss in view of improving capability for multiplexing/demultiplexing of optical signals.
An example of the technique for assuring low loss of an array waveguide grating, which has been proposed in the related art such as the U.S. Pat. No. 5,745,618 (particularly, FIG. 7), is illustrated in FIG. 2. The array waveguide grating of the related art is respectively provided with transition regions 71, 72 immediately adjacent to the boundary between the channel waveguide array (diffraction grating) 760 and the first slab waveguide 710 and to the boundary between the channel waveguide array 760 and the second slab waveguide 720. These transition regions 71, 72 are formed through orthogonal intersection of waveguide paths a1 to an (n is a positive integer) to the channel waveguide array 760.
Structures of the optical couplers in the related art are illustrated in FIG. 3A to FIG. 3C. FIG. 3A shows a structure of an ordinary star coupler. This star coupler is formed in the structure attained by cutting the structure of the array waveguide grating of FIG. 1 explained above at the center thereof. This single composition does not have the function for multiplexing/demultiplexing wavelength multiplexing optical signals but the function to coupling or decoupling optical signals. This star coupler is also requested to realize low optical transmission loss like the array waveguide grating explained above. FIG. 3B shows a structure attained by implementing the low loss technique which has been proposed as the related art for such a star coupler. In the case of this structure, a transition region 202 is provided immediately adjacent to the boundary between the output waveguide array 206 and the slab waveguide 200 like the array waveguide grating explained above. In addition, the similar low loss technique is also proposed for a splitter. Structure of this splitter is illustrated in FIG. 3C. This splitter is also provided with a transition region 302.
These transition regions are formed, as illustrated in FIG. 4, through orthogonal intersection of waveguide paths a1 to an to the waveguide array. Width W(ax)x=1 to n of the waveguide path is wide immediately adjacent to the slab waveguide but progressively decrease as it becomes further away from the slab waveguide. Meanwhile, the period Λ in formation of the waveguide path is constant.
In more practical, the width W(ax)x=1 to n of the waveguide path gradually decreases to W(an): 2 μm from W(a1): 18 μm as it becomes further away from the slab waveguides 710, 200. On the other hand, a separation gap between the adjacent waveguide paths (width of separation) W(sx)x=1 to n progressively increases to W(sn): 18 μm from W(s1): 2 μm as it becomes further away from the slab waveguides 710, 200. The relationship among the width W(ax) of the waveguide path, the separation gap W(sx), and the period Λ for arrangement of the waveguide paths satisfy the formula (1). The period Λ is 20 μm.W(ax)+W(sx)=Λ, x=1 to n  (1)
The channel waveguide array 760 (or the output waveguide array 206) is different to a large extent from the slab waveguides 710, 200 in the distribution of electric field of the light to be propagated (mode profile). Therefore, matching error of mode profile is generated in the area immediately adjacent to the boundary. Such matching error will cause an optical coupling loss. Accordingly, such matching error can be eased by eliminating abrupt change in the mode profile. As a result, optical coupling loss can be decreased.
In this related art, transition regions 71, 202 are provided. Width of these waveguide paths W(ax) progressively decreases as it becomes further away from the slab waveguides 710, 200. Therefore, the mode profile is changed step by step to avoid abrupt change thereof. Accordingly, matching error of mode profile is eased and optical coupling loss is lowered.
In this related art, it is explained that the period Λ is not required to be a constant and the width of the waveguide path is not required to be linearly decreased. However, it is explained that essential condition to attain the merits of the invention is that the width W(ax) of the waveguide path progressively decreases as it becomes further away from the slab waveguide.
As explained above, in the related art proposes realized low optical loss, the waveguide paths are provided at a part of the channel waveguide array immediately adjacent to the slab waveguide as illustrated in FIG. 2. Thereby, optical coupling loss is lowered. However, such optical coupling loss may be reduced than that in the ordinary array waveguide grating illustrated in FIG. 1 but a problem rises here, in which it is difficult to attain large reduction effect. The reason is that the waveguide path changes to narrow path from wide path as it becomes further away from the slab waveguide. The waveguide path has the effect to capture again the scattering light leaked between waveguides into the waveguide array. However, at the wider part of waveguide path, radiation of light from the waveguide array to the outside thereof through the waveguide path also occurs simultaneously. This phenomenon triggers the event that the scattering light captured by the waveguide path does not stay at the waveguide array and is radiated again to the outside of waveguide array via the waveguide path. As a result, a problem occurs here, that the scattering lights captured cannot be coupled within the waveguide array. Therefore, the structure of the array waveguide grating of the related art illustrated in FIG. 2 has a limit in the effect in realization of low loss.
As illustrated in FIG. 4, the waveguide paths are formed in the equal period in the structure of the related art. As a result, only the specific wavelength easily receives adverse effect. Accordingly, it may occur that the characteristic changes depending on every specific wavelength. Moreover, width of the waveguide path progressively decreases as it becomes further away from the slab waveguide. Therefore, an additional problem occurs here, in which shape management of each waveguide path becomes difficult in the manufacture and testing.
As illustrated in FIGS. 3A to 3C, a similar problem is also generated in the low loss technique for star coupler and splitter of the related art.